1. Field of the invention
The present invention relates to a magnetic field adjusting apparatus, a magnetic field adjusting method and a recording medium. More specifically, the present invention relates to a magnetic field adjusting apparatus, a magnetic field adjusting method and a recording medium containing a magnetic field adjusting program, for a magnetic field generator used in an MRI (magnetic resonance imaging) apparatus and so on.
2. Description of the Related Art
A magnetic field uniformity required of a magnetic circuit used in an MRI apparatus is very rigid (30 pmm for example). At a time of shipment from factory, an adjustment is performed to the magnetic field, so the magnetic field uniformity is assured, but vibration during transportation, environmental change at a place of installation and so on often deteriorate the magnetic field uniformity (to 50 pmm approx. for example). Therefore, when the MRI apparatus is delivered to the place of installation, the magnetic field uniformity is readjusted by using a movable yoke and an adjustment bolt. The magnetic field uniformity can be obtained by the formula (Magnetic field strength maximum valuexe2x80x94Magnetic field strength minimum value)xc3x97106/(Center magnetic field strength or Average magnetic field strength), and a smaller value indicates a higher magnetic field uniformity.
If the magnetic field uniformity cannot be brought back within a predetermined range in this adjustment, a final adjustment must be performed, in which magnetic field adjusting pieces each made of a small magnet in the form of e.g. a rectangular parallelepiped is bonded onto a silicon steel plate on a pole piece.
For this process, Japanese Patent Laid-Open No. 9-56692 discloses a technique of using a linear programming and so on for calculating the locations and the number of the magnetic field adjusting pieces to be bonded onto the pole piece.
However, no disclosure has been made for the specifics of the process, and the magnetic field adjustment is not easy.
It is therefore a primary object of the present invention to provide a magnetic field adjusting apparatus, a magnetic field adjusting method and a recording medium that make possible easy and accurate magnetic field adjustment.
According to an aspect of the present invention, there is provided a magnetic field adjusting apparatus for adjusting a magnetic field of a space in a magnetic field generator including a pair of plate yokes opposed to each other and a permanent magnet disposed on an opposed surface of each plate yoke. The apparatus comprises: means for measuring a magnetic field strength at a predetermined position in the space; means for storing in memory an amount of change in the magnetic field caused by placement of a magnetic field adjusting piece at a predetermined location of the magnetic field generator; means for inputting a target value for a magnetic field uniformity; means for calculating a location and the number of the magnetic field adjusting pieces based on the magnetic field strength, the amount of change in the magnetic field and the target value; means for calculating an expected magnetic field uniformity based on the location and the number of the magnetic field adjusting pieces; and means for outputting the location and the number of the magnetic field adjusting pieces if the expected magnetic field uniformity is not greater than a predetermined value.
According to another aspect of the present invention, there is provided a magnetic field adjusting method for adjusting a magnetic field of a space in a magnetic field generator including a pair of plate yokes opposed to each other and a permanent magnet disposed on an opposed surface of each plate yoke. The method comprises: a step (a) of measuring a magnetic field strength at a predetermined position in the space; a step (b) of storing in memory an amount of change in the magnetic field caused by placement of a magnetic field adjusting piece at a predetermined location of the magnetic field generator; a step (c) of inputting a target value for a magnetic field uniformity; a step (d) of calculating a location and the number of the magnetic field adjusting pieces based on the magnetic field strength, the amount of change in the magnetic field and the target value; a step (e) of calculating an expected magnetic field uniformity based on the location and the number of the magnetic field adjusting pieces; a step (f) of outputting the location and the number of the magnetic field adjusting pieces if the expected magnetic field uniformity is not greater than a predetermined value; and a step (g) of disposing the magnetic field adjusting piece on the magnetic field generator based on the outputted location and the number of the magnetic field adjusting pieces.
According to still another aspect of the present invention, there is provided a computer-readable recording medium containing a program for adjusting a magnetic field of a space in a magnetic field generator including a pair of plate yokes opposed to each other and a permanent magnet disposed on an opposed surface of each plate yoke. The program is executable by a computer and comprises: a step of inputting a magnetic field strength at a predetermined position in the space; a step of storing in memory an amount of change in the magnetic field caused by placement of a magnetic field adjusting piece at a predetermined location of the magnetic field generator; a step of inputting a target value for a magnetic field uniformity; a step of calculating a location and the number of the magnetic field adjusting pieces based on the magnetic field strength, the amount of change in the magnetic field and the target value; a step of calculating an expected magnetic field uniformity based on the location and the number of the magnetic field adjusting pieces; and a step of outputting the location and the number of the magnetic field adjusting pieces if the expected magnetic field uniformity is not greater than a predetermined value.
According to the present invention, a linear programming method for example is employed in the calculation of the location(s) and the number of the magnetic field adjusting pieces based on the magnetic field strength in the space of the magnetic field generator, the amount of change in the magnetic field caused by the placement of the magnetic field adjusting piece at the predetermined location of the magnetic field generator and the target value for the magnetic field uniformity. The expected magnetic field uniformity is calculated based on the locations and the number of the magnetic field adjusting pieces. The locations and the number of the magnetic field adjusting pieces for the expected magnetic field uniformity not greater than a predetermined value are outputted. The worker should simply dispose the magnetic field adjusting pieces based on the output, and therefore, the magnetic field can be adjusted easily and accurately in accordance with the inputted target value, with a small number of the magnetic field adjusting pieces.
Preferably, a check is made if the number of the magnetic field adjusting pieces given by the calculation is not greater than an upper limit or not. Then, the expected magnetic field uniformity expected for the number of the magnetic field adjusting pieces not greater than the upper limit is compared with the predetermined value. If the expected magnetic field uniformity (dispersion of magnetic field strength) is not greater than the predetermined value, the corresponding locations and the number of the magnetic field adjusting pieces are outputted. As described, by setting an upper limit to the number of the magnetic field adjusting pieces to be disposed, the magnetic field can be adjusted with a fewer magnetic field adjusting pieces, without interfering with a gradient coil.
Further, preferably, if the expected magnetic field uniformity is greater than the predetermined value, an expected magnetic field uniformity expected by further placement of the magnetic field adjusting piece at a predetermined location of the magnetic field generator is calculated, for each placement of the magnetic field adjusting piece. Then, a location and the number of the magnetic field adjusting pieces that minimize the expected magnetic field uniformity are outputted, and placement of the magnetic field adjusting piece is made on the magnetic field generator based on the outputted location and the number of the magnetic field adjusting pieces.
As described, if the expected magnetic field uniformity is greater than the predetermined value, an expected magnetic field uniformity is further calculated by using for example a direct search method, and the locations and the number of the magnetic field adjusting pieces that minimize the expected magnetic field uniformity are selected and outputted. Therefore, worker can adjust the magnetic field more accurately by disposing the magnetic field adjusting pieces onto the magnetic field generator based on the output.
According to another aspect of the present invention, there is provided a magnetic field adjusting apparatus for adjusting a magnetic field of a space in a magnetic field generator including a pair of plate yokes opposed to each other and a permanent magnet disposed on an opposed surface of each plate yoke. The apparatus comprises: means for measuring a magnetic field strength at a predetermined position in the space; means for storing in memory an amount of change in the magnetic field caused by placement of a magnetic field adjusting piece at a predetermined location of the magnetic field generator; means for calculating an expected magnetic field uniformity expected by placement of the magnetic field adjusting piece at a predetermined location of the magnetic field generator, based on the magnetic field strength and the amount of change in the magnetic field, for each placement of the magnetic field adjusting piece; and means for outputting a location and the number of the magnetic field adjusting pieces that minimize the expected magnetic field uniformity.
According to still another aspect of the present invention, there is provided a magnetic field adjusting method for adjusting a magnetic field of a space in a magnetic field generator including a pair of plate yokes opposed to each other and a permanent magnet disposed on an opposed surface of each plate yoke. The method comprises: a step (a) of measuring a magnetic field strength at a predetermined position in the space; a step (b) of storing in memory an amount of change in the magnetic field caused by placement of a magnetic field adjusting piece at a predetermined location of the magnetic field generator; a step (c) of calculating an expected magnetic field uniformity expected by placement of the magnetic field adjusting piece at a predetermined location of the magnetic field generator, based on the magnetic field strength and the amount of change in the magnetic field, for each placement of the magnetic field adjusting piece; a step (d) of outputting a location and the number of the magnetic field adjusting pieces that minimize the expected magnetic field uniformity; and a step (e) of disposing the magnetic field adjusting piece on the magnetic field generator based on the outputted location and the number of the magnetic field adjusting pieces.
According to still another aspect of the present invention, there is provided a computer-readable recording medium containing a program for adjusting a magnetic field of a space in a magnetic field generator including a pair of plate yokes opposed to each other and a permanent magnet disposed on an opposed surface of each plate yoke. The program is executable by a computer and comprises: a step of inputting a magnetic field strength at a predetermined position in the space; a step of storing in memory an amount of change in the magnetic field caused by placement of a magnetic field adjusting piece at a predetermined location of the magnetic field generator; a step of calculating an expected magnetic field uniformity expected by placement of the magnetic field adjusting piece at a predetermined location of the magnetic field generator, based on the magnetic field strength and the amount of change in the magnetic field, for each placement of the magnetic field adjusting piece; and a step of outputting a location and the number of the magnetic field adjusting pieces that minimize the expected magnetic field uniformity.
According to the present invention, the expected magnetic field uniformity is calculated by means of the direct search for example, based on the magnetic field strength in the space of the magnetic field generator and the amount of change in the magnetic field caused by the placement of the magnetic field adjusting piece at the predetermined location of the magnetic field generator, for each placement of the magnetic field adjusting piece. Then, the location(s) and the number of the magnetic field adjusting pieces that minimize the expected magnetic field uniformity are selected and outputted. The worker should simply dispose the magnetic field adjusting pieces onto the magnetic field generator based on the output, and thus the magnetic field can be adjusted easily and accurately.
Preferably, checking is made if the number of the magnetic field adjusting pieces placed on the magnetic field generator for the calculation of the expected magnetic field uniformity is not greater than an upper limit or not. If the number of the magnetic field adjusting pieces is not greater than the upper limit, then the locations and the number of the magnetic field adjusting pieces that minimize the expected magnetic field uniformity are outputted. By setting an upper limit to the number of the magnetic field adjusting pieces to be disposed, the magnetic field can be adjusted with a fewer magnetic field adjusting pieces, without interfering with a gradient coil.
In the above invention, preferably, a silicon steel plate is provided on the permanent magnet, and placement of the magnetic field adjusting piece is made on the silicon steel plate. With this arrangement, the amount of change in the magnetic field per magnetic field adjusting piece can be set to a small value, making easy to perform fine adjustment to the magnetic field.
Further, preferably, the magnetic field adjusting piece is provided by a magnet. This arrangement allows the magnetic field adjustment to be made in both the positive and the negative directions, making possible to perform the magnetic field adjustment more flexibly.
The above object, other objects, characteristics, aspects and advantages of the present invention will become clearer from the following description of an embodiment to be presented with reference to the accompanying drawings.